Powder metallurgical techniques allow for production of machine parts having complicated shapes with extremely high dimensional accuracy and are thus capable of significantly decreasing the production costs of such machine parts. Therefore, various machine parts produced by applying powder metallurgical techniques are used in many fields. Furthermore, in recent years, demand for miniaturization or reduced weight of machine parts has increased, and various precursor powders for powder metallurgy to produce small and lightweight machine parts having sufficient strength have been examined.
JP H01-219101 A (PTL 1), JP H02-217403 A (PTL 2), and JP H03-162502 A (PTL 3), for example, disclose precursor powders for powder metallurgy produced by adhering an alloying powder to surfaces of iron powder or alloy steel powder. Such powders mainly composed of iron (iron-based powder) are usually produced by adding an additive powder (for example, copper powder, graphite powder, iron phosphide powder, manganese sulfide powder, or the like) and a lubricant (for example, zinc stearate, aluminum stearate, or the like), and the resultant mixed powder is used in the production of machine parts.
The iron-based powder, additive powder, and lubricant, however, have different characteristics (shape, particle size, and the like), and thus flowability of the mixed powder is not uniform. Hence, the following problems (a) to (c) occur.
(a) The iron-based powder, additive powder, and lubricant unevenly distribute locally due to the influence of vibration or dropping during transport of the mixed powder to a storage hopper.
(b) Since relatively large spaces occur between particles of the mixed powder charged in the hopper, the apparent density of the mixed powder decreases.
(c) The apparent density of the mixed powder depositing in a lower portion of the hopper increases over time (i.e. under the influence of gravity), whereas the mixed powder stored in an upper portion of the hopper has a low apparent density. Therefore, the apparent density of the mixed powder is not uniform across the upper and lower portions of the hopper.
In other words, with conventional techniques, it is extremely difficult to mass-produce machine parts having uniform strength using mixed powder.
In order to solve the above problems (a) to (c), it is necessary to increase flowability of the mixed powder that includes the iron-based powder, additive powder, and lubricant.
To that end, JP H05-148505 A (PTL 4) discloses an iron-based powder mainly composed of an iron powder having a predetermined range of particle sizes. However, this technique not only decreases the yield of the iron powder, since an iron powder outside of the specified range cannot be used, but also causes difficulty in uniformly and sufficiently filling thin-walled cavities, such as a gear edge or the like, with the iron-based powder.
JP 2002-515542 A (PTL 5) discloses a technique for improving flowability at the time of warm formation by including 0.005% to 2% by weight of SiO2 having a particle size of less than 40 nm. This technique is problematic, however, in that SiO2 remains upon sintering and inhibits sintering between iron powder particles, thereby decreasing the strength of the resultant sintered body.
To address these problems, JP 2008-505249 A (PTL 6) discloses a method for increasing the flowability of a composition for powder metallurgy that includes an iron or iron-based metal powder, a lubricant, and/or a binder. With this method, 0.001% to 0.2% by weight of carbon black having a particle size of less than 200 nm and a specific surface area larger than 100 m2/g is added to the composition.
JP 2009-522446 A (PTL 7) discloses a composition for iron-based powder metallurgy that includes an iron powder or iron-based metal powder and a particulate composite lubricant, wherein the composite lubricant includes particles having a core that includes solid organic lubricating material, fine carbon particles being adhered to the organic lubricating material. This is a technique to mix iron powder with a lubricant having fine carbon particles on the surface thereof in advance before mixing the iron powder and the composite lubricant so as to achieve excellent flowability and to prevent agglomeration between the lubricants.
For the same purpose, JP 4379535 B2 (PTL 8) discloses an iron-based powder for powder metallurgy in which flowability improving particles that include 50 mass % to 100 mass % of carbon black are adhered to the surface of iron powder with a binder, the degree of penetration of the binder being in a range of 0.05 mm to 2 mm, the coverage of the iron powder by the binder being 10% or more and 50% or less and the coverage of the binder by the flowability improving particles being 50% or more.